A. Field of the Invention
The present invention relates generally to test valves, and more particularly to a valve for use in the system having a source of in-service pressure and a source of test pressure wherein the valve is shiftable automatically responsive to test pressure between an in-service position and a test position that maintains separation of in-service and test pressures and shifts automatically to the in-service position when test pressure is relieved.
B. Description of the Prior Art
There have been developed a number of systems for monitoring and protecting flow lines and pressure vessels. These systems include high-low systems, as for example the system described in U.S. Pat. No. 3,043,331, which are adapted to maintain flow through a flow line when the pressure within the line lies within a preselected range of pressures and close the line when the line pressure falls outside the selected range. These systems also include safety-relief systems which are adapted to vent protected flow lines or pressure vessels when the pressure exceeds a certain maximum. Examples of safety-relief valves are disclosed in Axelson, Inc. brochure AD64-233, March 1982, entitled "Pilot Operated Pressure Relief Valve", which generally include a relief valve which includes a dome and a valve element. The valve element has an effective area that is smaller than the effective area of the dome. Both the dome and the valve element are exposed to in-service pressure, which maintains the relief valve in a closed position. A control system is provided which is adapted to supply in-service pressure to the dome when the in-service pressure is less than the maximum. However, when in-service pressure exceeds the maximum, the control system is adapted to block in-service pressure and bleed the dome, thereby allowing the relief valve to open.
When it is necessary to set or test a pressure control in a safety-relief system, the in-service pressure must be disconnected from the control and a separate variable source of test pressure connected. Most commonly, testing is achieved by the use of two manually operated valves: an inlet valve to isolate in-service pressure from the control; and a test valve to connect test pressure with the control. The primary shortcoming of the two valve manual system is in the possible isolation of the control from the in-service pressure following testing, as would occur if the operator failed to reopen the inlet valve following a test. Such isolation would make the safety-relief system inoperative. A secondary, but important, shortcoming is in the accidental opening of the test valve prior to closing the inlet valve, which could result in venting in-service pressure to the atmosphere. A further shortcoming of prior test systems is that it may be desirable to test the operation of the control without opening the relief valve. In prior systems, whenever the control shifts to the block and bleed position, the relief valve opens.
There have been attempts to provide a test valve which eliminates the two valve manual system. For example, in U.S. Pat. No. 3,817,282, there is disclosed an in line test valve which includes a plunger element that is in a normally open position to allow the flow of line pressure to the sensors or control. The plunger includes a test pressure passage that is normally closed by an internal check valve. When it is desired to apply test pressure to the sensors, an operator or stinger is manually inserted into the valve to move the plunger valve element to seat and isolate the sensors or control from line pressure. The stinger additionally mechanically opens the check valve to allow the flow of test pressure to the sensors or control. The valve of the '282 patent does not entirely overcome the shortcomings of the two valve manual system. First, the valve of the '282 patent does not return automatically to the in-service position after test pressure is removed. Rather, the valve remains in the test mode with the sensors or control isolated from in-service pressure until the stinger is removed. Additionally, as the stinger operates the plunger valve element between the closed test position and the fully open in-service position, the check valve remains open, thereby allowing communication between the line pressure and the test pressure.
Another stinger operated test valve is disclosed in disclosed in U.S. Pat. No. 3,987,817. In the valve of the '817 patent, the check valve is eliminated, but the valve still remains in the test position until the stinger is removed. Likewise, during operation between the closed test position and the fully opened in-service position, the valve allows communication between line pressure and test pressure.
Accordingly, it is an object of the present invention to provide a test valve that will shift back and forth between the in-service and test positions without a mechanical operator. It is a further object of the present invention to provide a test valve that is operable to shift between the in-service and test conditions responsive to test pressure. It is a further object of the present invention to provide a test valve that will shift automatically from the test position to the in-service position when test pressure is removed. It is a further object of the present invention to provide a test system that automatically isolates the relief valve actuator from the control during testing but, which automatically reestablishes control after testing.